Entries in start/stop system
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When the new A8 debuts on July 11, it will be the first A8 to offer a standard hybrid powertrain. In this case, a 48-volt belt alternator starter (BAS) system aimed at increasing running refinement, performance and efficiency.The so-called “mild hybrid” unit has two major components: the water-cooled 48-volt BAS, and a trunk-mounted lithium-ion battery that stores and releases the energy it creates. A conventional 12-volt pinion-drive starter also is used, but only for cold starts. Though the BAS is capable of producing the torque necessary to turn over a cold engine, the belt drive could slip when transferring this level of energy to the engine.

The BAS has near-instantaneous response, and can quickly restart the engine in changing situations. Since the hybrid unit is designed to shut the engine off while coasting or when coming to a stop, the BAS must have the ability to quickly restart the engine with minimal fuss, and be ready to add full power if necessary. In addition, at speeds between 34 and 99 mph, the BAS allows the new A8 to enter a noiseless coasting mode for up to 40 seconds, producing zero emissions, and swiftly restart the engine when the driver depresses the throttle. Start/stop operation is active from 14 mph.

Unlike previous stop/start systems, the A8’s system can discern different operating parameters and act accordingly. For example, if the car ahead drives off while the A8 is still stopped and the driver has his foot on the brake, the system restarts so the vehicle is ready to flow into traffic. Also, the powertrain controller is capable of processing route data and information from the A8’s sensors and cameras to decide whether the unit should be in recuperation or coasting mode. As a result, it can decide to recover energy in order to slow the vehicle so as not to contact the vehicle ahead while in coasting mode. Audi claims the mild hybrid drive system can reduce fuel consumption by 0.7 liters per 100 kilometers. — CAS